Optical filters are important devices in optical fiber communications systems. Monolithic optical waveguide filters are particularly promising because they can perform complex circuit functionalities and because they can be made by mass production integrated circuit techniques.
The requirements of optical filters vary with applications. Many applications require a rectangular wavelength response in order to maintain a low-loss and wavelength-independent transmission in a passband and a high-level rejection to all wavelengths in a stopband. For example, anticipated telecommunications applications seek a 1.3/1.551 .mu.m WDM filter a flat and low-loss passband at 1.280-1.335 .mu.m and a-50 dB stopband at 1.525-1.575 .mu.m. Another desired application is a gain equalization filter to flatten the gain of an Er-doped fiber amplifier chain. This requires an equalization filter with an amplitude response which is essentially the inverse of the amplifier gain.
Various devices have been proposed to fill these new, demanding requirements but none are fully satisfactory. Multilayer thin-film filters can be used to construct optical filters in bulk optics, but they are undesirable because they cannot be readily integrated and because of difficulties in coupling light to and from fibers. Mach-Zehnder (MZ) interferometers have been widely employed, but they have a sinusoidal response, giving rise to strongly wavelength-dependent transmission and a narrow rejection band. Other designs have encountered a variety of practical problems. Accordingly, there is a need for a new type of monolithic optical waveguide filter.